Heating of molten metal

ABSTRACT

D R A W I N G

n. c. BONE 3,741,751

HEATING OF MOLTEN METAL Filed Sept. 10, 1970 June 26, 1973 A 4 o nf United States Patent 3,741,751 HEATING 0F MOLTEN METAL David Charles Bone, Quainton, England, assignor to The British Aluminium Company Limited, London, England Filed Sept. 10, 1970, Ser. No. 71,155 Claims priority, application Great Britain, Sept. 12, 1969, 45,163/69 Int. Cl. C22b 9/00, 29/00, 21/06 US. CI. 75-65 8 Claims ABSTRACT OF THE DISCLOSURE RELATED APPLICATION This application is related to commonly assigned copending application Ser. No. 71,112, which is a continuation-in-part of Ser. No. 835,872, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to improvements in the heating of liquid metal, particularly but not exclusively molten aluminium and especially moten aluminium under cleaning treatment and intended for the production of ingots for working.

It is well known that liquid aluminium contains varying amounts of gas and non-metallic inclusions, and that their presence may give rise to defects in finished products. Many procedures have been proposed for the removal of the gas and inclusions. Thus the gas content may be reduced to an acceptable level by bubbling chlorine, nitrogen or argon through the melt or by treatment of the metal with hexachlorethane. The use of chlorine and hexachlorethane gives rise to a fume disposal problem necessitating expensive equipment, whereas with the nitrogen treatment as heretofore proposed, the metal becomes contaminated through formation of non-metallic inclusions.

It is common experience, and in accordance with the teaching of US. Pat. No. 2,821,472, that metal which has been degassed in a reverberatory furnace by conventional chlorine treatment begins to take up gas as soon as the chlorine treatment has been completed and the gas burners have been turned on. Furthermore British Pat. No. 435,104 teaches the need for maintaining an atmosphere free from hydrogen, water vapour, or other hydrogen containing gas above the metal surfaces when carrying out degassing operations, and British Pat. No. 569,619 teaches that such an atmosphere is still required even when degassing is being carried out using nitrogen.

Thus direct heating from above by gas flame would appear to be ruled out since not only would the products of combustion, rich in water vapour, pass over the metal surface, but the metal would further be agitated in this atmosphere of steam, so that, far from removing gas, it would be expected that additional hydrogen would be picked up by such a procedure.

In copending patent application Ser. No. 71,112 there is described a process for removing non-metallic constituents from liquid aluminium and its alloys in a continuous manner which comprises flowing liquid metal through a containing vessel, passing a substantially inert gas into the metal in the vessel, and maintaining a liquid flux cover over the liquid metal in the containing vessel; the metal may subsequently be flowed through a device comprising a plurality of flux-lined channels. Two kinds of equipment are described for carrying out the invention of said copending patent application namely (i) a box lined with refractory bricks and heated by immersion heaters each consisting of a refractory sheath of silicon carbide or nitride within which is situated a gas burner, the refractory sheaths being partly immersed in the liquid metal and (ii) a crucible which is heated by a gas burner which is placed externally to the crucible.

The brick-lined equipment is very convenient for use where very high metal flow rates, e.g. exceeding 1000 lb./min., are involved, but this type is expensive to operate because of the short life of the refractory sheaths of the immersion heaters and the need to construct the gas burners from heat resisting nickel alloys, the hot products of combustion returning back up the sheaths and past the burners on their way to atmosphere.

It would be desirable to heat the metal electrically from above but this would be costly except in locations where cheap electrical energy may be available.

SUMMARY OF THE INVENTION I have carried out experiments with top heating, by direct flame, of metal contained in a refractory lined brick box equipment and covered with a layer of flux and have endeavoured to carry out the process of said copending application therewith but without flowing the metal continuously through the equipment. I first made vacuum solidification tests by taking samples from the metal bath using a hand ladle and found these to contain much gas as expected. I found however on closer investigation that the gas was not in the metal in the box but was picked up during the few seconds that the sample of metal in the hand ladle was passing through the products of combustion whilst being lifted out of the box. On turning off the burners it was found that the metal underneath the flux layer gave no bubbles when a sample was subjected to the vacuum solidification test.

I have thus made the surprising discovery that although the presence of a liquid flux layer on the metal surface is no bar to the egress of gas from the metal to atmosphere and in fact facilitates the transfer, it does not facilitate the pick up of gas by the metal even when the metal is being agitated by nitrogen treatment in an atmosphere rich in water vapour. For gas to be picked up by the metal it is necessary for the metal to react with the steam to produce atomic hydrogen and aluminium oxide, and the liquid salt flux layer not only provides a protective film on the metal surface but will also quickly wet and absorb any oxide produced which would otherwise restrict the egress of hydrogen from the metal.

I have now applied this result to the problem of heating large refractory lined box units for carrying out the process of said copending application and I have found that it is possible to carry out by means of gas burners all the heating which is required to operate a unit large enough to treat metal at a rate of over 1000 lb./min. In this way very substantial economies have been made in the operating cost of equipment of this size.

It is desirable to place a refractory lined lid or hood over the metal surface so that, as the refractory becomes hot, heat is transferred by radiation to the metal surface as well as by contact with the gaseous products of combustion.

I have further found that the maintenance of a small flow of nitrogen through the diffuser tubes even when the equipment is not in use is beneficial in maintaining a uniform metal temperature.

If desired the gas burners may be turned off during part or all of the cast provided the thermal insulation of the unit is suflicient to enable the desired temperature to be maintained, but good results have been obtained with the burners in operation during casting.

If desired the flux cover may be removed during periods when casting is not in progress in order to promote 3 heat transfer between the combustion products and the metal.

Accordingly the present invention provides a process for the heating of liquid aluminium and aluminium alloys in which the metal is agitated by passage of an inert gas therethrough, a liquid salt flux cover is provided on the metal surface, and the metal is heated from above by a combustion system, the gaseous products of which are in direct contact with said flux.

The gas burners for carrying out the present invention do not need to be constructed from special heat resisting nickel alloys. They may conveniently be fixed to removable heat insulating lids and fire across the metal surface. Alternatively the combustion system may be such as to produce a flat disc of flame above the metal surface or small jets which heat radiant panels placed over the metal surface. A wall hugger flat flame burner producing a hollow cone of flame is also suitable.

The operation of the present invention is further illustrated by way of example by reference to the accompanying drawings and the following illustrative example.

FIG. 1 is a schematic sectional view of an apparatus for degassing and cleaning molten aluminium, and

FIG. 2 is a view similar to that of FIG. 1 showing an alternative heating system.

A brick lined box 1 is divided by a baflle wall 2 into chambers A and B. The molten metal enters the chamber A under a baflle 3 which serves to confine a liquid salt flux layer 4 on the surface of the metal in chamber A and prevent it from flowing backwards along the launder 5 to the furnace tap hole 6. A refractory lined lid 7 is placed over the ingoing chamber A. In FIG. 1 a row of gas burners, one of which is seen at 8, provide the heat required, Whilst in FIG. 2 heating is carried out by means of the wall hugger flat flame burner 9. A gas insert to the metal is introduced through pipes 10 at the bottom of chamber A. The metal passes under the baflle Wall 2 into the chamber B and thence into a casting launder 11.

As described in said copending application preferably the residence time of the aluminium or its alloys in the containing vessel is at least 1 minutes and preferably 3 or more minutes, the inert gas (for example nitrogen, argon, carbon monoxide or carbon dioxide) is passed at a rate sufiicient to provide at least 10 (preferably 30) cu. ft. per ton of liquid metal and at least one pound (preferably 2-3 pounds) of flux per 100 square inches of liquid metal is provided.

The temperature of the aluminium during treatment should be in the range 675-800 C., 700-750 C. being preferred.

The flux should be substantially free from oxides, oxysalts and fluosilicates and from volatile halides. It should consist essentially of chlorides and fluorides of the alkali and alkaline earth metals including magnesium and should be thinly fluid at the melting point of the metal. When melted the flux should have a lower density than that of the metal. Fluxes consisting of NaCl, KCl, and NaF, (r cryolite) may be used where introduction of sodium is not objectionable, but it is preferred to limit the NaF content to by weight or the cryolite content to 8% by weight. Use of CaCl is undesirable since the flux is then rendered hygroscopic with no compensating advantage. For certain alloys Where the sodium content must be limited in order to avoid cracking during rolling a flux containing MgCl is desirable. Suitable flux compositions for use with the invention are shown in Table I.

TABLE I.-SUITABLE FLUX COMPOSITIONS (PERCENT BY WEIGHT) 4 EXAMPLE Using equipment of the type shown in FIG. 1 with 5 gas burners, each consuming gas at a rating of 150,000 B.t.u.s per hour, untreated remelt metal containing gas and inclusions was caused to flow through the equipment at speeds ranging from 350 to 800 lb./min. The equipment was operated intermittently over a period of 2 weeks, during which time no heating was applied to the unit other than by means of the gas burners shown. The metal processed through the unit was gas free as judged by the vacuum solidification test operating at a pressure of 2-5 torr. In most of the tests the burners were kept in operation whilst casting through the unit was in progress.

I claim:

1. A process for the cleaning and degassing of liquid aluminium and aluminium alloys comprising passing an inert gas through the liquid metal to simultaneously degas and agitate said liquid metal, providing a liquid salt flux cover on the metal surface, the improvement comprising heating said metal from above by a combustion system, the gaseous products of which are in direct contact with said flux.

2. A process as claimed in claim 1 in which the inert gas is nitrogen.

3. In a process for removing non-metallic constituents from liquid aluminium and its alloys in a continuous manner comprising flowing the liquid metal through a containing vessel, passing a substantially inert gas into the metal in the vessel to simultaneously degas and agitate the liquid metal and maintaining a liquid flux layer on the metal in the containing vessel, the added step of heating the liquid metal in the container substantially solely from above by a combustion system, the gaseous products of which are in direct contact with said flux.

4. A process according to claim 3 in which the residence time of the metal in the containing vessel is at least one and a half minutes, the inert gas is passed at a rate sufficient to provide at least ten cubic feet per ton of liquid metal and at least one pound of flux is provided for each 100 square inches of surface area of liquid metal being treated.

5. A process according to claim 3 in which the flux consists essentially of the chlorides and fluorides of the alkali and alkaline earth metals including magnesium.

6. A process according to claim 3 in which the inert gas is nitrogen.

7. A process according to claim 3 in which the containing vessel is covered by a heat insulating lid under which the gaseous products circulate.

8. A process according to claim 3 in which the containing vessel is divided by a battle wall into an ingoing chamber and an outgoing chamber, the inert gas is passed through the ingoing chamber and the flux layer and gaseous products are maintained over the metal in the ingoing chamber.

References Cited UNITED STATES PATENTS 2,768,075 10/ 1956 Sterental -68 R 3,159,478 12/1964 Gottschalk 7593 2,840,463 6/1958 Stroup 75-68 3,172,757 3/ 1965 Hess 75-93 3,281,238 10/ 1966 Bachowski 75-68 R 3,123,467 3/ 1964 Peters 7568 R RICHARD 0. DEAN, Primary Examiner P. D. ROSENBERG, Assistant Examiner US. Cl. X.R. 75-67, 68 

